I have sky downstairs and i have one aerial cable coming down feeding the TV an anologe signal, can i feed the Sky signal back up the same cable (both cables will always be connected on a loop at the end of the cable) up to a splitter and back to a portable tv?
If the cable is an RF (coaxial) cable, like for ariels, then you should be able to do this, but obviously only showing the same on the tv upstairs as is coming out the sky box downstairs. If your sky box has both an RF (ariel) out and a Scart out, plug it into your downstairs tv with scart and feed the anologue cable into the sky box. Then try and tune the sky box in upstairs, just like tuning in another channel or video player.
Yes you can feed it back up BUT if you are feeding the terestrial feed to the downstairs TV you might have problems with crossover interference. Use the satelite feed through the downstairs TV and use a gender changer to feed the RF out to your TV upstairs and tune it in as per previous, an alternative would be to purchase a "video sender" available from Maplins and many other "Dixons" type stores, just a thought